Here we defend the view that one ought to categorize and classify at least some mental disorders as clusters of interrelated dysfunctions of (usually, several) cognitive capacities – that is, the kinds of capacities that are postulated in cognitive science; capacities that are understood as entities that are primarily individuated in cognitive-functional terms (CF-systems); systems that have a set of peculiar properties in their manner of operation when processing information or representations. Usually, some of the mental disorders postulated in psychiatry (...) are clinically heterogeneous with respect to their symptoms, and tend to be comorbid – that is, one disorder is often accompanied by other disorders – for example, schizophrenia often comes with major depression. We argue that describing the manner in which CF-systems operate within themselves and with other CF-systems as well as with stimuli and other kinds of information can best explain why certain mental disorders are very heterogeneous with respect to their symptoms, and why comorbidity is so frequent among these disorders. A consequence is that both within-category heterogeneity and comorbodity are not necessarily indications of defective classifications but are rather a consequence of the actual cognitive functional architecture itself and are therefore to be expected in these cases. (shrink)